The present invention relates to an optical transmitter and a code conversion circuit used therefor, and more particularly to an optical transmitter of a high-speed optical communication system and a code conversion circuit used therefor.
Recently, a high-speed optical communication system that transmits at an ultra high-speed more than a transmission rate of 10 Gb/s using a long-distance optical fiber has been developed in haste. In the case where the system must be developed before the improvement of the high-speed characteristics of an electronic device, the high-speed characteristics of an electronic circuit hardly catches up with the high-speed characteristics required for an optical transmission circuit.
FIG. 1 is a block diagram of an example of an output part of a conventional optical transmitter. In the same figure, a plurality of series of low-speed electronic signals and a clock are supplied to a serializer 10 so as to obtain a high-speed electronic signal and a clock. The high-speed electronic signal and the clock are supplied to a D-type flip flop 14 of an optical transmission circuit 12 where waveform shaping is performed, and thereafter, supplied to a drive circuit 16 so as to drive an optical modulator 18 and are converted to a high-speed optical signal. The optical modulator 18 turns on a light in response to a value 1 of data and turns off the light in response to a value 0, for example. The high-speed optical signal is transmitted to an optical fiber 20. In this case, the bit rate of the optical signal is determined by the bit rates of the serializer 10, the D-type flip flop 14 and drive circuit 16 of the optical transmission circuit 12, which are the circuits handling the electronic signal. It is impossible to speed up the bit rate of the optical signal more than the working speed of these circuits that handle the electronic signal.
Incidentally, in Japanese Laid-Open Patent Application No. 3-200923 etc., the applicant has proposed an optical transmission circuit as shown in FIG. 2, which converts a plurality of series of low-speed electronic signals to a high-speed optical signal by using a both-side electrode Mach-Zehnder type optical modulator. In FIG. 2, a first low-speed electronic signal is synchronized with a clock in a D-type flip flop 22 and supplied to a drive circuit 26. In addition, a second low-speed electronic signal is synchronized with the clock in a D-type flip flop 24, delayed by a delay circuit 28 only for a half cycle T/2 when the data cycle of each of the low-speed electronic signals is T, and supplied to a drive circuit 30. A both-side electrode Mach-Zehnder type optical modulator 32 is driven by each of the drive circuits 26 and 30, and generates and transmits, to an optical fiber 34, a high-speed optical signal in which the first and second low-speed electronic signals are multiplexed. In this case, the bit rate of the optical signal is twice that of each of the electronic signals.
Here, a description will be given of a case where two series of low-speed electronic signals are supplied and parallel-serial conversion is performed thereon. A low-speed electronic signal (a1, a2, . . . ) indicated by FIG. 3(A) and a low-speed electronic signal (b1, b2, . . . ) indicated by FIG. 3(B) are supplied to the serializer 10, and a high-speed electronic signal (a1, b1, a2, b2, . . . ) indicated by FIG. 3(C) is obtained. The optical modulator 18 performs optical modulation using the high-speed electronic signal, and a high-speed optical signal indicated by FIG. 3(D) is transmitted.
On the other hand, in the optical transmission circuit in FIG. 2, in a case where a first low-speed electronic signal (a1, a2, . . . ) indicated by FIG. 4(A) and a second low-speed electronic signal (b1, b2, . . . ) indicated by FIG. 4(B) that have the same wave forms as those in FIGS. 3(A), (B) are supplied to the flip-flop 22, 24, respectively, the output of the delay circuit 28 is delayed only for a half cycle T/2 as indicated by FIG. 4(C). The both-side electrode Mach-Zehnder type optical modulator 32 outputs a high-speed optical signal indicated by FIG. 4(D) that is Exclusive-OR of the signal of FIG. 4(A) and the signal of FIG. 4(B). It should be noted that, in FIG. 4(D), xe2x80x9c+xe2x80x9d represents Exclusive-OR operation.
By the way, the optical transmission circuit shown in FIG. 2 is for a case where the first and second low-speed electronic signals have the same phase. However, in a case where the second low-speed electronic signal is delayed from the first low-speed electronic signal for the half cycle T/2, as shown in a block diagram of FIG. 5, the second low-speed electronic signal is supplied to a D-type flip flop 25 that latches with an inversion clock and supplied to the drive circuit 30 without delaying the output. In FIG. 5, those parts that are the same as those corresponding parts in FIG. 2 are designated by the same reference numerals, and a description thereof will be omitted.
As described above, in the circuit of FIG. 1, the input low-speed electronic signals are simply bit-multiplexed and the high-speed optical signal indicated by FIG. 3(D) is output. While in the circuit of FIG. 2, the signal obtained by performing Exclusive-OR operation on the input first and second low-speed electronic signals is output as the high-speed optical signal. Since the above-described two circuits have different patterns of the high-speed optical signal, it has not been possible to use the optical transmission circuit in FIG. 2 instead of the optical transmission circuit 12 shown in FIG. 1.
For example, it is conceivable to prepare two serializers 10 shown in FIG. 1 and obtain a further higher speed optical signal by supplying the high-speed electronic signals output by each of the serializers to the flip flops 22, 24 of the optical transmission circuit in FIG. 2. However, in this case, there was a problem in that the Exclusive-OR of the output signals of the two serializer is output as an optical signal, and an optical signal that is obtained by simply bit-multiplexing the output signals of the two serializers cannot be output.
It is a general object of the present invention to provide an optical transmitter that can obtain a simply bit-multiplexed optical signal by using an optical transmission circuit employing a both-side electrode Mach-Zehnder type optical modulator and transmit a high-speed optical signal having a twice bit rate with respect to electronic signals, and to provide a code conversion circuit used therefor.
In order to achieve the object, the present invention is configured to include a code conversion circuit that is supplied with first and second signals having a first bit rate, and generates and outputs, through Exclusive-OR operation, third and fourth signals from which signals obtained by simply bit-multiplexing the first and second signals and having a bit rate twice the first bit rate are obtained, and an optical transmission circuit that performs optical modulation by a both-side electrode Mach-Zehnder type optical modulator using the third and fourth signals, and outputs an optical signal that is Exclusive-OR of the first and fourth signals.
According to such an optical transmitter, the output optical signal is Exclusive-OR of the third and fourth signals, that is, a signal obtained by simply bit-multiplexing the first and second signals and having a twice bit rate, and thus it is possible to transmit a high-speed optical signal having a twice bit rate with respect to electronic signals.